The purpose of this study is to determine the influence of the rheological properties of blood on microcirculatory and tissue function in normal circumstances and in disease. In our previous studies we focused on the processes by which red cell aggregation affects the function of the microcirculatory network, particularly that of the venous microcirculation. For the coming grant period we will expand our studies to consider more broadly how the rheological properties of blood influence function of the circulatory system. One aspect will be to consider the process of phase separation of red cells and plasma as it occurs in the microcirculatory vessels. Available experimental and theoretical findings suggest that the cell-free layer may significantly influence microvascular regulation as a determinant of wall shear stress and NO release by the endothelium, as well as the degree of NO scavenging by hemoglobin in the red cell core of the flow stream. We will determine the wall shear stress in microcirculatory vessels by dual micropressure measurements and examine shear stress during variations in cell free layer width, hematocrit and flow rate. We will investigate the potential for determining wall shear stress principally from optical measurements. We will directly measure the effect of variations in cell free layer width, wall shear stress and scavenging capacity of the red blood cells on NO levels in microcirculatory vessels. At the capillary network level we will study the effects of phase separation on the fraction of capillaries with red cell flow (functional capillary density, FCD), and O2 delivery to tissues. A major emphasis of our studies will be to examine how changes in the flow properties of blood in disease states affects the function of the microcirculation and in turn the tissues that they support. We will examine the effects of anemia, polycythemia, increased red cell rigidity as occurs in septicemia and other pathophysiological states. Our aim is to develop an integrated view of the contribution of biophysical factors to microhemorheology and how these microrheological properties affect the regulation of the microcirculation and its function both in health and disease. In relation to public health, these studies will provide a greater appreciation of how the physical properties of the red cells are changed in disease states and how these changes in turn affect the function of the body.